SAN MATEO — As doctors and families across the state struggle with an epidemic of whooping cough, county health officials have announced a free clinic for residents to be vaccinated against the contagious disease.

As of Aug. 31, eight times as many cases of the disease had been reported as at the same time last year, according to San Mateo County Health System spokeswoman Robyn Thaw. That was double the increase the state witnessed in late June; three infants have been killed in the epidemic since then as well. A total of eight infants have died statewide since the epidemic first took hold, but none were in San Mateo County.

In the same time period — from the beginning of the year through Tuesday — the county saw 61 cases of the disease reported, Thaw said.

Many parents are under the mistaken impression that one vaccination against whooping cough, known officially as pertussis, is good for a lifetime protection against the disease, Thaw said. Most children inoculated against the disease before kindergarten need a booster shoot between the ages of 10 and 12, and adults should get a booster every 10 years, she said.

Whooping cough is highly contagious and causes severe coughing attacks, Thaw said. The disease can last for months and is often mistaken in early stages for a common cold or flu.

The free clinic will be held from 10 a.m. to 2 p.m. Sept. 18 in Room 100 at the Health Administration Building, 225 37th Ave., San Mateo. Residents age 7 and older are encouraged to get vaccinated, especially if they are in close contact with infants who are too young to get the shot. No appointments are necessary, and residents will not be asked for ID.

Contact Sean Maher at 650-348-4473.

For more information about whooping cough and about the free vaccine clinic, visit www.smhealth.org/pertussis or call (650) 573-2346.

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